The present invention relates to a method of inspecting a phase-change type optical recording medium, and more particularly to an inspection method of evaluating degradation of quality of a regenerative signal regenerated from a phase-change type optical recording medium due to repetition of overwrite in a high temperature environment.
Heretofore, in a phase-change type optical recording medium (hereinafter referred to as “an optical disc”), a recording layer made of a material in which a phase change between a crystalline state and an amorphous state is caused due to a difference in applied laser power is formed on a transparent substrate. During a recording operation, a laser beam is applied to the recording layer to partially melt the recording layer, and the partially molten recording layer is rapidly cooled and solidified, thereby forming an amorphous recording mark in the recording layer. On the other hand, during an erase operation, the laser beam is applied to the amorphous recording mark to heat the amorphous recording mark at a temperature which is equal to or lower than a melting point of the recording layer, but equal to or higher than a crystallization temperature of the recording layer. As a result, the amorphous recording mark is crystallized to return the phase state of the amorphous recording mark back to an original unrecorded phase state. The repetitive rewrite, i.e., the so-called repetitive overwrite can be performed for the phase-change type optical disc by repetitively executing these kinds of processing.
However, when the overwrite is repetitively performed for the optical disc, a thickness of the recording layer is changed due to mass transfer in the recording layer and materials in a dielectric layer contacting the recording layer diffuse into the recording layer. As a result, there is encountered such a problem that the jitter characteristics become worse, and the signal amplitude is reduced, and thus the signal error rate increases.
In order to solve this problem, a technique is disclosed in which a metallic layer is provided right above or right under a recording layer in order to cause the heat generated within the recording layer to diffuse into the metallic layer, whereby the thermal damages incurred on a recording layer during an overwrite operation is reduced to enhance the durability of the recording layer against the repetition of the overwrite (refer to Japanese Patent Laid-Open No. Hei 6-36352).